The goal of this project is to define the molecular mechanisms involved in the replication of enveloped RNA viruses and in particular, to understand factors which influence the regulation and expression of viral genetic information. The possible role of the viral RNA genome in directing virus assembly is currently being investigated in a system which uncouples murine leukemia virus RNA synthesis from terminal steps in virus maturation. The virus particles produced under these conditions resemble deletion mutants since they lack the 70S RNA genome and contain only low molecular weight RNA species, predominantly 4S RNA. Our recent results demonstrate that the 4S RNA in the defective virions represents a selected population of host tRNAs, as is the case for normal particles. Thus, the viral RNA genome does not appear to control the selective packaging of host tRNAs which occurs during virus assembly. A possible role for one or more viral proteins in the tRNA selection process will be explored with other types of defective virions.